Synopsis Two pilots and two passengers departed from a logging camp on Fish Egg Inlet, British Columbia, in the float-equipped de Havilland DHC-6-100 aircraft for a 16 nautical mile charter flight to Pruth Bay. As the aircraft was climbing through 100feet above ground level, an elevator control cable broke. The aircraft pitched to a nose-up attitude, stalled, and descended to the surface of the water. The captain, who was seriously injured, escaped from the sinking wreckage; the first officer and the two passengers drowned. The aircraft was destroyed by impact forces and sank. The Board determined that the down elevator control cable failed at station 376 due to corrosion, and, as a result, the pilot lost control of the aircraft. The corrosion was not detected by the maintenance personnel during the aircraft's last cable inspection. 1.0 Factual Information 1.1 History of the Flight The first officer, who occupied the left pilot seat and was handling the controls, taxied the aircraft a short distance away from the dock and took off. The captain occupied the right pilot seat. As the aircraft climbed through 100feet, and just as the flaps were retracting, the aircraft pitched up violently and stalled. The pilots were unable to regain control of the aircraft, and it crashed into the water about 100feet from the shore and sank immediately. The captain, who sustained serious injuries from the crash, escaped from the wreckage and swam to shore. The first officer and the two passengers remained in the aircraft and drowned. When the aircraft failed to arrive at Pruth Bay, personnel from Pacific Coastal Airlines (the company) notified the Rescue Coordination Centre and started their own search. A search pilot spotted debris in the water near the logging camp at about 1430 Pacific daylight saving time (PDT)(2) that day. He landed to investigate, found the captain on the shore nearby, and transported him to the Port Hardy hospital. The accident occurred during daylight hours at approximately 1148PDT, at latitude 5136'N and longitude 12741'W. There were no witnesses, other than the aircraft captain, to this accident. 1.2 Injuries to Persons 1.3 Damage to Aircraft 1.4 Other Damage 1.5 Personnel Information 1.5.1 Captain and First Officer The captain and the first officer were certified and qualified for the flight in accordance with existing regulations. 1.5.2 Quality Manager The company's Port Hardy maintenance base Quality Manager had been employed by the company as such since 1992. He began his career as an aircraft maintenance engineer (AME) in 1975 and he had worked for a number of employers on a wide range of aircraft types, including the DHC-6 in 1980. The company's Maintenance Control Manual (MCM) states the following: The Quality Manager is responsible to the President for the quality and regulatory compliance of all aircraft maintenance work performed at the Quality Manager's base and to the Director of Maintenance for day to day maintenance tasks. Duties of the Quality Manager include ensuring that all the work done in the shops or on the aircraft is carried out in accordance with applicable standards, practices, and specifications, and ensuring this manual and all technical publications are amended in a timely manner as required. At the time of the occurrence, the Quality Manager at the Port Hardy base was responsible for supervising four licensed AMEs and one apprentice AME. 1.5.3 Aircraft Maintenance Engineer The AME who completed the most recent inspection of the station 376 cable group on the accident aircraft was employed as a senior AME at the company. He had been with the company for about eight years at the time of the accident. He was certified and qualified to conduct the Equal Maintenance for Maximum Availability (EMMA) number 18 inspection in accordance with existing regulations. The AME had about 15 years of experience working on the DHC-6 aircraft, including the EMMA number 18 inspection. He had read the de Havilland bulletins relating to corrosion, and TAB (Technical Advisory Bulletin) 680/1 relating to previous elevator control cable failures, when they first arrived. However, he did not have a clear recollection of their contents. 1.6 Aircraft Information The weight and centre of gravity were estimated to have been within the prescribed limits. Documentation indicates that the aircraft was certified and equipped. 1.7 Meteorological Information 1.8 Wreckage and Impact Information 1.9 Medical Information 1.10 Survival Aspects The occupants were not wearing the available flotation devices, nor were they required to by regulation. It was not determined whether their use would have prevented the occupants' drowning. All aircraft crew and passenger seats were equipped with seat-belts, which were used by the occupants. Although the aircraft crew seats were equipped with shoulder harnesses, neither the captain nor the first officer was wearing one. 1.11 Aircraft Maintenance 1.11.1 Inspection Program Section 4 of the company MCM states in part that all aircraft are maintained to the inspection program for that type. An Inspection Program Approval (IPA) for this aircraft type was approved by Transport Canada (TC) on 06June 1994. The program is described as follows: The DHC-6 Twin Otter is maintained as per the manufacturer's developed maintenance program, P.S.M. 1-6-7. The program consists of 48 checks of 100 flight hours each, for a complete cycle of 4800 hours. Each numbered check, from 1 to 48, lists the required inspection to be carried out, on form P.S.M. 1-6-7 (F1) This aircraft is being maintained in accordance with the provisions of the Airworthiness Manual. Periodic inspections are performed in accordance with the standards of Chapter 571 and this approved inspection program. All work is performed in accordance with the manufacturer's maintenance manuals or other data that is acceptable or approved by Transport Canada (TC). The manufacturer's developed maintenance program is the TC-approved de Havilland Equal Maintenance for Maximum Availability (EMMA) inspection system. In this system, a number of individual work cards are specified for every 100-hour inspection, each addressing a separate area of inspection on the aircraft. EMMA work card 18 addresses the aft fuselage and empennage area of the aircraft. This is the location of the cable group at station 376. Under Inspection, item 6 states: ... [inspect] control cables for fraying, broken strands, flattening, corrosion and security of turnbuckles and cable ends; plastic sheathing, where applicable, for cracking and deterioration. NOTE: It is important to operate controls through full range so that cables move away from pulleys and all portions of cables are exposed for inspection. Inspection of aircraft control cables is common to almost all aircraft inspections, and, to a large degree, falls into the category known as standard practices. The inspection of control cables is routine, and all licensed AMEs would be very familiar with the task. In addition, de Havilland maintenance manual PSM 1-6-2 contains a section that provides brief, general instructions on the inspection of control cables. In the Twin Otter, only the most aft portion of the pulley group is visible from the aft fuselage. The control column must be moved in a fore and aft direction by another person at the front of the aircraft when the cables are being inspected so that a length of cable on both sides of the pulley can be viewed. It is necessary to remove the baggage compartment floor adjacent to station 376 to view both forward and aft sides of the pulley group. In the Preparation section, EMMA work card number 18 does not specify the removal of the baggage compartment floor panel that allows access to the forward side of the station 376 pulley cluster. In accordance with EMMA procedures, the cable inspection is required every 800 hours for aircraft in normal service, and every 400 hours whenever a cargo of livestock or corrosive material is regularly transported. The cables had no fixed service life. 1.11.2 Aircraft Maintenance History The company took possession of this aircraft, in accordance with existing regulations on 28May 1994 at Red Lake, Ontario, and ferried it to Port Hardy. It arrived with 20,667.4 hours total time since new (TTSN). The maintenance records received from the previous owners and operators were not complete. Consequently, the installation date(s) of the control cables could not be determined. Maintenance records kept by the company subsequent to the purchase of the aircraft were complete. The company completed an acceptance inspection on 06June 1994, in Port Hardy. The aft baggage area floor was removed and the area was pressure washed; the control cables were lubricated and the floor was reinstalled. Other maintenance included painting the aircraft exterior, replacing the cabin interior, and replacing a cracked rudder pedal. Additionally, the area under the baggage compartment floor was cleaned and treated with a corrosion-preventive substance, and corroded screws were replaced with stainless steel screws. On 28June 1994, at aircraft time 20,708 hours TTSN, the maintenance required by EMMA work card number 17 was completed. This was the first EMMA inspection completed after the aircraft was purchased by Pacific Coastal Airlines. The maintenance required by EMMA card number 18 was completed by three company AMEs on 15August 1994, at aircraft time 20,802 TTSN, while the aircraft was floating at the company's Port Hardy floatplane base. The AME who inspected the aft fuselage could not recall details of his inspection of the control cables located at aircraft station 376. He did recall, however, that another engineer had moved the controls for him while he examined the cables, and that he had lubricated the cables with a common corrosion-preventive lubricant. He also recalled inspecting the de-ice boot ejectors and recording the ELT number; these items are located at the rear of the aft fuselage. The aircraft had 20,870 hours TTSN at the time of the accident. This was about 68 hours since the last cable inspection, and 202.6 hours since arriving at Pacific Coastal's base in Port Hardy. 1.12 Elevator Cable Failure A microscopic examination of the broken cable revealed that individual strands had been significantly reduced in cross-section, and that most of the strands appeared to have been broken for some time. The cables had been immersed in sea water during the period that the aircraft was submerged and all exhibited the effects of salt water corrosion. The up elevator cable and both rudder cables that passed through the station 376 pulley cluster exhibited evidence of corrosion; broken strands were discovered on two of the cables. TSB Engineering Branch Report LP 149/94 concluded as follows: The elevator cable failed as a result of gradual deterioration due to corrosion in an area which was in contact with the lowermost pulley of a pulley cluster. As the corrosion progressed, individual wires and strands started failing and the wire ends caused paint removal in the surrounding bracket. Contamination of the physical evidence by exposure to sea water precluded an objective evaluation of the corrosion mechanism. It is believed that moisture collected on the pulley and provided the catalyst for corrosion to take place. Several similar instances of control cable failures were reported with this aircraft [type]. The uncorroded portion of the cable was tested and found to satisfy the manufacturer's specifications [the generic material specification for this cable]. The report did not make an estimate of the time frame over which the corrosion took place. A corrosion-resistant stainless steel cable was available as an optional alternative to the carbon steel cable. The stainless steel cables are used by some DHC-6 operators and are specified by de Havilland for some applications; however, they are prone to increased wear and require replacement more frequently than the carbon steel cables. Stainless steel cables were not a requirement for this aircraft. 1.13 Company Audit History Three audits of the company were undertaken by TC between 1989 and the accident date. In addition, at the company's request, TC System Safety Directorate completed a safety review of the company during that time. During an audit conducted in August 1989, the audit team found some non-conformance items with respect to airworthiness. The company was maintaining sea-planes, based at Port Hardy, by a method that was not described in their Maintenance Control Manual. Additionally, the company's copy of the Air Regulations and Air Navigation Orders did not have current amendments. During an audit conducted in 1991, the audit team found 42 non-conformance findings in airworthiness areas. In a summary letter regarding the audit, the audit team concluded as follows: ... the audit resulted in a number of non-conformance findings which indicate that both flight and airworthiness operations are not being conducted to a satisfactory standard. The company is not in compliance with the approved policies, procedures and control systems which provide guidance to company personnel. The deficiencies in operational control, maintenance management and quality control are below the standard required of an Operating Certificate holder. ... a program of increased inspection and surveillance will be undertaken by Transport Canada Inspectors to monitor company progress in these areas. All of the non-conformance findings were subsequently addressed by the company. The most recent audit of company operation and maintenance facilities, prior to the occurrence, was conducted by TC in October 1992. This audit included the areas of flight operations, flight watch/dispatch, passenger safety, transportation of dangerous goods, flight safety, airworthiness, and security. The audit team concluded as follows: Pacific Coastal Airlines is making improvements in many aspects of operations and maintenance, but still requires a dedicated effort to establish administrative documented procedures in the Flight Operations and Maintenance Control Manuals. The company submitted a corrective action plan to TC, as was required. 1.14 Additional Information The float supplement in the DHC-6 Flight Manual specified a flap setting of 20 degrees for take-off. The company made it a practice to operate with a take-off flap setting of 30 degrees in order to improve take-off performance. De Havilland Inc. and Bombardier Regional Aircraft Division state that the use of 30 degrees of flap for take-off on any Twin Otter aircraft floatplane is not approved and that such a flap setting will reduce that aircraft's single engine climb capability to zero, or a negative number depending upon aircraft gross weight and ambient conditions.